This invention relates to etchants used in manufacturing processes and, more particularly, to etchants for removing metal from glass substrates without attacking the glass.
In the manufacture semiconductor devices, several etching steps are generally performed. Consequently, various types of etching operations are well developed arts in the semiconductor manufacturing industry. However, with the current growth of optoelectronic products, such as photoncouplers and optically coupled solid-state relays, new etching problems are facing semiconductor design and process engineers. Specifically, glass substrates are frequently used to support optoelectronic components. In conjunction with the step of mounting a semiconductive device pellet on the glass, metallic conductive paths are usually applied to the glass to provide coupling to the pellet. Typically the conductive paths are formed by coating the glass with metal and etching unwanted portions of the metal from the glass. When manufacturing optically coupled devices, a constraint new to the semiconductor device designer must be considered. Specifically, the glass must not be attacked in such a way as to become pitted or hazy or otherwise lose its transparency. Heretofore, no entirely satisfactory process was available.
It is now felt that a primary reason for the unavailability of satisfactory etchants is that those etchants that are commonly used are of the oxidation-reduction type in which an oxidizing agent causes oxidation of the material to be etched and a reducing agent destroys and removes the oxide. Further, the commonly used reducing agents, such as hydrofluoric acid, also attack the glass. Thus, whenever the glass is exposed, it is attacked. The overall etching process of the prior art cannot be controlled well enough to assure absolute uniformity of etching. Therefore, the metal is removed from some areas more rapidly than from other areas. The glass under the first exposed areas is attacked while the remaining etching is completed. This effect is compounded by the need to be absolutely certain that etching is complete over all areas of the surface inasmuch as if etching were prematurely terminated and a thin film of metal remained on the glass, short or leaky circuits among the conductors could occur and optical efficiency could be impaired.
An object of this invention, therefore, is to provide an etchant that effectively and completely removes selected metals from glass without attacking glass.